Research Of Microwave Absorbing Material FeSiAl Alloy Used In Linac High Power Coaxial Load

Collinear load is a novel remnant power load for linear accelerators. Microwave-absorbing material is an essential part in S-band high power (>10kW) collinear load R&D. According to the power level and operation conditions of the collinear load, the material is expected to have high microwave dissipating capability, low outgassing rate, good thermal conductivity and processability, meanwhile the bonding force to the substrate is sufficient. A kind of FeSiAl alloy is selected as the coating material through elaborative investigations. The researches of the coating microscopic structure, vacuum quality, thermal properties and electromagnetic characteristics are carried out afterwards, laying a solid foundation for subsequent high power collinear load R&D.The microscopic appearance of the FeSiAl coating is observed by SEM, and the phase composition is analyzed by XRD. The porosity, density and outgassing rate are also tested. Thermal properties of the FeSiAl like thermal expansion coefficient, specific heat capacity and thermal conductivity are measured, considering the cooling system design of the cavity and thermal stress analysis of the coatings. The thermal expansion coefficient results reveal to be comparable with that of the copper substrate, thus higher thermal stress could not emerge to cause coating flaking off. The comparison of the two sets of thermal conductivity experiments not only approves the measurement accuracy of the thermal conductivity, but also validates the intermediate quantity tests of the density and specific heat capacity.In order to obtain the dielectric constant and permeability of the FeSiAl at 2856 MHz, the coaxial translation/reflection method is adopted to investigate the electromagnetic parameters taking advantage of the VNA (vector network analyzer), at the same time, CST Microwave Studio is utilized to perform test simulations. Theory analysis and CST simulations both indicate that the gaps between the test samples and the fixture would influence the experiment results seriously. To eliminate the air-gap effect, samples of the FeSiAl powder mixed with paraffin are molded with different percentages, and subsequent tests gain stable results. The electromagnetic parameters of the FeSiAl are extracted finally through the equivalent formulas of mixed medium. Researches demonstrate that the equivalent formulas of Bruggeman, Looyenga and QCACP are more suitable for the case of the FeSiAl powder.